The Evolution of Physicochemical and Gas Transport Properties During Thermal Rearrangement of a Polyhydroxyamide (PHA) – the Effect of Conversion, Thermal History and Crosslinking

The evolution of physicochemical and gas transport properties of a thermally rearranged polybenzoxazole derived from a polyhydroxyamide (PHA) have been systematically investigated as a function of temperature in the present work. It is revealed that thermal cyclization temperature plays the other determining factor during the rearrangement process since not only could it affect the degree of thermal conversion, but also influences thermal history and introduces thermal crosslinking. By thermally cyclizing the PHA precursor membranes at different temperatures in the range of 300 – 450 °C, similar degrees of conversion (~ 91 %) have been achieved, while enhanced gas separation properties are found in the membrane thermally cyclized at a higher temperature. Analyses based on membrane physical properties and glass transition behavior prove that the aforementioned phenomenon is attributed to the suppression of polymer chain segmental rotation and the lower chain packing density induced by a higher thermal cyclization temperature. It is noticed that cyclodehydration reaction during thermal rearrangement has triggered the occurrence of thermal crosslinking by homolytic cleavage of the hydroxyamide linkage. Possible decarboxylation induced thermal crosslinking mechanism has been proposed and described in details. The gas sorption and diffusion characteristics of CO₂/CH₄ further elucidate that the amplification of membrane fractional free volume has greater impact on diffusivity selectivity than solubility selectivity. The dual-mode sorption data confirm that the transformation of gas transport properties is attributed to the changes in both Henry's molecular dissolution and Langmuir sorption, which are closely related to the membrane physical properties.